A nonaqueous secondary battery, which is represented by a lithium ion secondary battery, has a high energy density and is widely used as a main electric power source of a portable electronic equipment, such as a portable phone and a notebook computer. The lithium ion secondary battery is demanded to attain a further high energy density, but has a technical issue on assuring safety.
A separator plays an important role on assuring safety of a lithium ion secondary battery, and under the current situation, a polyolefin microporous membrane, particularly a polyethylene microporous membrane, is used since it has a high strength and a shutdown function. The shutdown function referred herein means a function of shutting down an electric current by closing the pores of the microporous membrane when the temperature of the battery is increased, and the battery's generating heat is suppressed by the function, thereby preventing the battery from suffering thermal runaway.
The energy density of the lithium ion secondary battery is being increased year by year, and for assuring safety, heat resistance is demanded in addition to the shutdown function. However, the shutdown function contradicts the heat resistance since the operation mechanism thereof depends on closure of the pores through melting of polyethylene. There have been proposals on improvement in heat resistance with the molecular weight of polyethylene, the crystalline structure or the like, but sufficient heat resistance has not yet been attained. Such techniques have been proposed that polypropylene is blended or laminated, but under the current situation, these systems fail to attain sufficient heat resistance.
Separately, Patent Documents 1 and 2 and the like propose a separator having a porous layer formed of a polymer having sufficient heat resistance and a polyethylene microporous membrane laminated on each other for attaining both heat resistance and shutdown function. The structures of the heat resistant porous layer in these systems are roughly classified into two modes. In one mode, nonwoven fabrics formed of a heat resistant polymer or paper are laminated, and the techniques therefor are specifically disclosed in Patent Documents 1 and 2. In the other mode, porous layers formed of a heat resistant polymer are laminated by a wet coagulation method, and the techniques therefor are specifically disclosed in Patent Documents 3 to 14.
Patent Document 1 is an article obtained by laminating a polyphenylene sulfide nonwoven fabric and a polyethylene microporous membrane, and Patent Document 2 is an article obtained by laminating aramid paper and a polyethylene microporous membrane. The systems obtained by laminating a nonwoven fabric or paper with a polyethylene microporous membrane have an effect of preventing short circuit at a high temperature, but are difficult to maintain the shutdown state (the state where the separator has large resistance) owing to the large pore diameter of the heat resistant porous layer. This is because the polyethylene microporous membrane is locally melted down in the pores of the heat resistant porous layer, and in view of the standpoint, it cannot be said that they have sufficient heat resistance. Furthermore, they are difficult to have a decreased thickness with the current techniques since the nonwoven fabric and the paper are constituted by fibers, and are difficult to be applied to a nonaqueous secondary battery separator.
Patent Documents 3 to 14 are examples that disclose techniques of forming a heat resistant porous layer on a polyethylene microporous membrane by a wet coagulation method. In these techniques, a polyethylene microporous membrane, which is used in the current lithium ion secondary batteries, is generally used as a base, but when the heat resistant porous layer is formed on the polyethylene microporous membrane by the wet coagulation method for imparting sufficient heat resistance, such problems may occur that the shutdown characteristics are lowered (e.g., the shutdown temperature is shifted to a high temperature, and the resistance upon shutdown is lowered), and the membrane resistance of the separator is considerably increased to deteriorate the rate characteristics.
The major factor of the problems occurring is that a coating liquid used for forming the heat resistant porous layer intrudes into the pores of the base, as disclosed in Patent Document 15. Specifically, it is considered that the coating liquid used for forming the heat resistant porous layer intrudes into the pores on the surface side of the base, whereby the shutdown function, which works by closure of the pores of the base as the principle, is impaired, and the base is clogged at the interface to the heat resistant porous layer to increase the membrane resistance of the separator. Patent Document 15 discloses such a technique that a base is impregnated with a solvent in advance before forming a porous structure by the wet coagulation method with the coating liquid, thereby preventing the pores of the base from being clogged, but the method is not preferred due to the complicated operation. Furthermore, it cannot be said from the examples thereof that good shutdown characteristics are obtained.
Patent Document 16 discloses a structure that is free of the aforementioned clogging. What is disclosed is a structure of a separator obtained by simply laminating a low temperature contracting microporous membrane (i.e., a layer having a shutdown function) and a high temperature contracting microporous membrane (i.e., a heat resistant layer) without adhesion. The structure is obtained by simple lamination of two kinds of separators each having separated functions, and thus has no adhesion interface between the two kinds of separators. Accordingly, it is considered that a good shutdown function can be attained, and the resistance can be suitably lowered. However, it is necessary to handle two sheets of separators simultaneously upon producing a battery, which brings about a problem of complicated operation upon producing the battery. Furthermore, the separators each having separated functions are not adhered to each other, and the low temperature contracting porous membrane is melted down (broken) at a high temperature. Because of the high temperature contracting porous membrane, it is considered that the short circuit between the positive and negative electrodes can be avoided by making the strength of the membrane suitable, but there is a problem of failing to maintain the shutdown state.
Patent Document 1: JP-A-2000-108249
Patent Document 2: JP-A-2006-054127
Patent Document 3: JP-A-2000-100408
Patent Document 4: JP-A-2001-023600
Patent Document 5: JP-A-2001-266949
Patent Document 6: JP-A-2002-190291
Patent Document 7: JP-A-2003-040999
Patent Document 8: JP-A-2004-349146
Patent Document 9: JP-A-2002-355938
Patent Document 10: JP-A-2005-209570
Patent Document 11: JP-A-2006-027024
Patent Document 12: JP-A-2006-289657
Patent Document 13: JP-A-2006-307193
Patent Document 14: JP-A-2006-273987
Patent Document 15: JP-A-2001-023602
Patent Document 16: JP-A-2003-059477